1. Field of the Invention
The present invention relates to a method and apparatus for measuring an optical wavelength of a coherent light source for use in a lightwave interferometer that obtains various mechanical measures.
2. Description of the Related Art
To measure a wavelength of a coherent light source, a heterodyne method is generally used to detect a beat signal caused from an interference between a reference light with a known wavelength and another light with an unknown wavelength, which is subjected to testing (hereinafter referred to as xe2x80x9ctest lightxe2x80x9d). When two coherent lights with slightly different wavelengths are mixed to interfere with each other, the resultant beat signal has a frequency equal to a difference between frequencies of the two lights. Thus, if the wavelength of one light is previously known, the wavelength of the other light can be measured.
The wavelength measurement by the heterodyne method requires the two wavelengths to have extremely close values. For example, in order for suppressing a beat signal detected to have a frequency in an order of MHz, a difference between the wavelengths of two lights used is required to maintain 1 ppm or less. This causes a disadvantage that makes it impossible to adapt the heterodyne method to such a light that has a greatly different wavelength from that of the reference light.
The use of a variable wavelength coherent light source, such as a dye laser and a semiconductor laser, allows a variety of traceable lightwave interferometry that can not be achieved by a fixed wavelength light source. If the variable wavelength of the semiconductor laser and the like, for use in such the measurement, is an assumed value, it lacks traceability. Therefore, a technology for calibrating the wavelength of the variable wavelength semiconductor laser is preferable to employ a reference light such as a stabilized He-Ne laser to perform a precise calibration. The conventional heterodyne method, however, can not perform such the calibration.
The present invention has been made in consideration of the above situation and accordingly has an object to provide a method and apparatus for measuring an optical wavelength, capable of measuring a wavelength of a coherent light source having a wide range of wavelengths, using a reference light.
The present invention is provided with a method of measuring an optical wavelength using an interference optical system. The interference optical system splits an input light beam in two, gives a predetermined optical path length difference to the resultant two light beams and thereafter synthesizes them to generate interference fringes. The method comprises the steps of: introducing a reference light with a known wavelength xcex1 from a coherent reference light source into a first interference optical system; introducing a test light with an unknown wavelength xcex2 from a coherent test light source into a second interference optical system; modulating interference fringes obtained from the respective interference optical systems by giving the same variation to the respective optical path length differences for the reference and test lights in the first and second interference optical system; and computing the wavelength xcex2 of the test light by detecting degrees of modulation of the respective interference fringes and using a ratio between the degrees and the wavelength xcex1 of the reference light.
Unlike the heterodyne method that subjects the test light into a direct interference with the reference light, according to the present invention, the same optical path length variation is given both to the test and reference lights in the first and second interference optical systems to modulate the received signals of interference fringes, obtained from the test and reference lights, in accordance with their wavelengths. Through the use of this fact, the wavelength of the test light can be computed by detecting the degrees of the modulations.
Thus, the reference light is not required to have a wavelength close to that of the test light, allowing the wavelength measurement to be performed within a wide wavelength range.
Specifically, the first and second interference optical systems may preferably share components so as to configure a single interference optical system. In this case, the reference and test lights are introduced via separate optical paths into the single interference optical system.
In addition, there are several modes for modulating the reference and test lights through modulation of the optical path length differences, and for demodulating the modulated received signals and computing the wavelength as below:
(a) In a first mode, the respective received signals obtained from the reference and test lights are amplitude-modulated in accordance with wavelengths thereof by giving an increasing or decreasing displacement to the optical path length differences at a constant velocity as the variation given to the optical path length differences for said reference and test lights.
In this case, frequencies f1 and f2 of intensity variations of amplitude-modulation received signals obtained from the reference and test lights are detected, and the wavelength xcex2 of the test light is computed from xcex2=(f1/f2)xcex1.
(b) In a second mode, the respective received signals obtained from the reference and test lights are phase-modulated in accordance with wavelengths thereof by giving a sinusoidal wave vibration having a predetermined amplitude of d and an angular frequency as the variation given to the optical path length differences for the reference and test lights.
In this case, phase-modulated received signals from the reference and test lights are PM demodulated to detect phase terms, xcexa81=(2xcfx80d/xcex1)+xcfx861 and xcexa82=(2xcfx80d/xcex2)+xcfx862 (where xcfx861 and xcfx862 are initial phase magnitudes), and a ratio between amplitudes of the phase terms, xcex2/xcex1, is computed to obtain the wavelength xcex2 of the test light.
(c) In a third mode, the respective received signals obtained from the reference and test lights are frequency-modulated in accordance with wavelengths thereof, by giving a sinusoidal wave vibration having a predetermined amplitude of d and an angular frequency of xcfx89 both on optical path length differences for the reference and test lights, and simultaneously superimposing an interfered beat signal with an angular frequency of xcfx89c on the received signals obtained from the reference and test lights in an acoustic optical modulator (AOM) disposed on the optical paths.
In this case, frequency-modulated received signals from the reference and test lights are FM demodulated, using a carrier frequency of xcfx89c, to detect phase terms, xcexa81=(2xcfx80d/xcex1)+xcfx861 and xcexa82=(2xcfx80d/xcex2)+xcfx862 (where xcfx861 and xcfx862 are initial phase magnitudes), and a ratio between amplitudes of the phase terms, xcex2/xcex1, is computed to obtain the wavelength xcex2 of the test light.
The sinusoidal wave vibration may be given to the optical path length differences for the reference and test lights through electrical modulation by an electric optical modulation (EOM) as well as a mechanical method that gives a signal to a mirror and the like in the interference optical system using a piezoelectric device and the like.
The present invention is also provided with an apparatus for measuring an optical wavelength using an interference optical system. The interference optical system splits an input light beam in two, gives a predetermined optical path length difference to the resultant two light beams and thereafter synthesizes them to generate interference fringes. The apparatus comprises a first interference optical system for receiving a reference light with a known wavelength xcex1 from a coherent reference light source; a second interference optical system for receiving a test light with an unknown wavelength xcex2 from a coherent test light source; a modulator for modulating interference fringes obtained from the respective interference optical systems by giving the same variation to the respective optical path lengths for the reference and test lights in the first and second interference optical system; a first photoreceptive device for receiving interference fringes output from the first interference optical system; a second photoreceptive device for receiving interference fringes output from the second interference optical system; and a wavelength calculator for computing the wavelength xcex2 of the test light, by detecting degrees of modulations of the received signals obtained from the reference and test lights at the first and second photoreceptive devices and using a ratio between the degrees and the wavelength xcex1 of the reference light. In this case, preferably the first and second interference optical systems may share components so as to configure a single interference optical system, and the reference and test lights are preferably introduced via separate optical paths into the single interference optical system.
The modulator and the wavelength calculator for processing the output from the modulator to compute the wavelength may also be configured specifically in accordance with the modes (a)-(c) above described. Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.